This invention relates to a thermal recording apparatus with which an image corresponding to image data is formed on a thermal recording material (hereunder referred to as a "thermal material") using a thermal head.
Thermal materials such as thermal films comprising a thermal recording layer on a film substrate are commonly used to record images produced in diagnosis by ultrasonic scanning. This recording method eliminates the need for wet processing and offers several advantages including convenience in handling. Hence, the use of the thermal image recording system is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, the thermal recording apparatus uses the thermal head having a glaze in which heat generating resistors corresponding to the number of pixels of one line are arranged in one direction and, with the glaze a little pressed against the thermal recording layer of the thermal material, the thermal material is relatively moved in a direction approximately perpendicular to the direction in which the heat generating resistors are arranged, and the respective heat generating resistors of the glaze are heated in accordance with the image data to be recorded to heat the thermal recording layer imagewise, thereby accomplishing image reproduction.
When image recording is effected using specified image data of the same recording density, the image formed has unevenness in the recording density due to the thermal head. Termed "shading", this unevenness in density is generally such that the density is the highest in the image area corresponding to the center of the thermal head in which the glaze extends and it decreases gradually toward either end of the image. In order to solve this problem of uneven density, shading correction is commonly performed in thermal recording.
To realize shading correction, the following procedure is taken: an image is recorded using specified image data of the same recording density; the density of the recorded image is measured by an optical means such as a densitometer, whereby shading correction data which corrects the image data in such a way that the image to be recorded will have a uniform density is calculated on the basis of both the recording density of the image data which is to be delivered from the thermal recording apparatus and the actually measured density of the recorded image; and the data of the image to be recorded is corrected using the thus calculated shading correction data.
Since the shading generally occurs due to the thermal head in the thermal recording apparatus, the originating site of uneven densities is not variable; however, the intensity of density unevenness varies with many factors such as the recording density of image data, the temperature of the thermal head, the image recording speed (the transport speed of the thermal material relative to the thermal head), the temperature and humidity of the thermal material and its gradient. Therefore, it is difficult to achieve shading correction in high precision.
To mention one example, the recorded images become more uneven in density if the temperature of the thermal head or the image recording speed increases.
This reduction in the precision of shading correction results in the deterioration of the quality of finished images and, particularly in medical areas where high-quality images need be recorded, the defect can potentially cause a serious problem by leading to a wrong diagnosis.